Semiconductor laser diodes are efficient sources of laser radiation, however the highly divergent beam emitted from a semiconductor laser diode presents problems in many applications. The divergence of the semiconductor laser diode's beam is caused by its exit aperture which is very narrow along one axis (the "fast" axis, which is defined to be perpendicular to the laser junction), and much wider along the perpendicular axis (the "slow" axis, which is defined to be parallel to the laser junction). These two axes correspond to the Y and X axes, as will be later explained. The cross section of the beam emitted along the fast, or Y, axis is highly divergent due to diffraction effects. In comparison, the wider aperture, defined along the X axis, emits a beam cross section that diverges only slightly.
Laser diodes, or more properly, semiconductor lasers, are generally constructed according to well-known principles of semiconductor manufacturing technology. A discussion of these principles can be found in Richard R. Shurtz II, Semiconductor Lasers and LEDs in Electronics Engineers' Handbook, 3rd ed. (hereinafter "Shurtz") (Donald G. Fink and Donald Christiansen, eds. 1989).
To correct the divergence of the output beam from a laser diode, one particularly successful methodology has been to direct that beam through a particularly configured cylindrical microlens, and several patents and patent applications are directed to furthering this technology.
One such device of an early type is taught in U.S. Pat. No. 4,731,772, as referenced in U.S. Pat. No. 5,050,153
Another method for the fabrication of cylindrical microlenses of the type embodied in the present invention is taught in U.S. Pat. No. 5,155,631. According the '631 reference, a preferred method for fabrication of cylindrical microlenses starts by forming the desired shape as a glass preform. The preform is then heated to the minimum drawing temperature of the glass, and a fiber is drawn from it. The cross-sectional shape of the fiber bears a direct relation to the shape of the preform from which it was drawn, thus forming the polished microlens. During the drawing process, the lens surfaces so formed become optically smooth due to fire polishing.
In order to collimate the beam produced by a laser diode, the invention taught in U.S. Pat. No. 5,081,639 teaches the mounting of a cylindrical lens optically aligned with the laser diode to provide a beam of collimated light from the Y axis of the diode. The laser diode assembly taught therein includes a diffraction-limited cylindrical lens having a numerical aperture greater than 0.5 which is used to collimate a beam from a laser diode. A collimated beam is one which is neither converging nor diverging; i.e., the rays within the beam are travelling substantially parallel to one another.
U.S. Pat. No. 5,181,224 illustrates the use of cylindrical lenses to (inter alia) create a slowly diverging beam light. This lens may be said to be "circularizing" and, when installed on any of a variety of laser diodes is available as the "CIRCULASER.RTM." diode available from Blue Sky Research in San Jose, Calif.
In U.S. patent application Ser. No. 08/837,002, entitled "MULTIPLE ELEMENT LASER DIODE ASSEMBLY INCORPORATING A CYLINDRICAL MICROLENS", there is described another diode/microlens system in which the microlens does not correct for astigmatism of the diode beam, but which is instead corrected downstream with a larger lens or other means. In this system, no active alignment is required to position the microlens adjacent to the laser diode facet, so automation of the process is rendered possible. However, other means are then required to correct for the astigmatism of the beam. These other means take the form of additional optical elements inserted into the beam emerging from the microlens.
In U.S. patent application Ser. No. 08/837,004, a laser diode/cylindrical microlens assembly is taught in which a crossed pair of cylindrical microlenses is attached to a substrate on which is mounted a laser diode chip. The microlenses are mounted with their flat surfaces facing the emitting facet of the diode, which arrangement provides for passive alignment and possible automated mounting, but requires no additional lenses for astigmatism correction. The crossed pair of lenses can collimate or focus the laser diode beam, for example focusing the beam into a single mode fiber.
U.S. Pat. No. 5,050,153 teaches a device related to the device taught in the '772 teaching. In this teaching, the device is implemented as a semiconductor laser optical head assembly utilizing a tilted plate for astigmatism correction in place of the cylindrical lens taught in the '772 reference.
To overcome the loss of optical efficiencies inherent in each of these designs, U.S. Pat. No. 5,181,224 utilizes a cylindrical microlens which with one optical element circularizes and corrects the astigmatism in the output beam of a semiconductor laser diode. To obtain these advantages, the cylindrical lenses must be aligned to tolerances less than 2 microns along at least one axis. This precision alignment has heretofore required the active alignment of the lens with the diode. The resultant apparatus, e.g., the previously discussed CIRCULASER.TM., is a low-divergence, low numerical aperture, highly efficient semiconductor laser diode assembly, with properties unmatched by other laser diodes.
Indeed, the advantages accruing to the CIRCULASER.TM. are only obtainable by the use of microlenses. In optical systems of the type described in U.S. Pat. No. 5,080,706, reducing the size of the optical elements thereof is generally regarded as having positive advantages in lens fabrication and accuracy. Indeed, the performance provided by the use of microlenses, i.e. lenses not substantially larger than about 1000 microns in diameter, is not attainable using macroscopic lenses.
U.S. patent application Ser. No. 08/725,151, entitled: "ELECTRO-OPTICAL DEVICE WITH INTEGRAL LENS", teaches an improvement to prior active alignment methodologies for mounting a long cylindrical microlens to a row of laser diode chips which has been cleaved from a wafer. According to this reference, the lens and the diode row are first positioned on a substrate. The long lens is then actively aligned to the diode row by powering at least one of the diodes and actively aligning the lens by inspection of the resultant laser beam formed in conjunction with the lens. Once aligned, the lens and diode are then fixed to establish their relative position. Finally, the lens/diode row is cut into individual diode chips. In this manner, the alignment "cost" is spread over a number of devices. While the methodology taught in this reference presents a substantial economy over the individual active alignment hitherto required, there still exists the need to actively align the diode row and the microlens.
While the previously discussed laser diode assemblies are fully effective for their intended use, the method of their manufacture has heretofore resulted in manufacturing inefficiencies. In any optical system, the alignment of the various optical elements is critical to the functioning of the system. This is certainly the case where a cylindrical microlens is incorporated into an optical system with a laser diode to provide a low-cost source of collimated light. As is typical of many optical applications, there are six degrees of freedom inherent in the positioning of the lens with respect to the laser diode, as shown in FIG. 1. Having reference to that figure, a cylindrical microlens, 100 is shown. The lens has three axes, X, Y and Z. The Z axis, 1, corresponds to the optical axis of the optical system. The X, 3, axis is transverse to the Z axis, 1, in the horizontal plane. The Y, 2, axis is also perpendicular to the Z axis but in the vertical direction. Positioning the lens along the X, Y, and Z axes defines the first three degrees of freedom. Furthermore, the lens may be rotated about each of these axes as shown at 10, 20, and 30, and each of these rotations also defines a degree of freedom with regard to alignment of the lens in the optical system. For cylindrical lenses, placement of the lens along the X axis, 3, is often not critical. In summary, the accurate alignment of a cylindrical microlens with respect to a semiconductor laser diode often requires precise alignment of one with the other with respect to five degrees of freedom.
One reason that alignment is required between lenses and other optical elements is that minor variations in lens geometry and size require correction. Moreover, the positional tolerances required to achieve optimal optical performance are exceptionally small: often less than two microns. This is particularly true along the Z axis, where positional accuracy is more often critical than along the Y axis, for instance. The achievement of this accuracy in positional alignment has heretofore generally been accomplished by some means of active alignment. With respect to microlenses, the act of heat-pulling the preform to draw the lens to its final size can produce microlenses which are either slightly over-pulled or under-pulled. Lenses having these "pulling errors" may very well present optical properties which are scaled versions of the intended lens. This presents a problem in prior attempts at passive alignment in that very small differences in physical dimension require different placement of the lens with respect to one or more degrees of freedom to effect proper alignment of the lens with respect to the diode or other device. While even a perfect lens has heretofore required some type of active alignment, pulling errors have absolutely mandated such a process.
A fairly typical active alignment methodology generally proceeds as follows: First, a section of cylindrical microlens is mounted on a small mounting bracket which because of its resemblance to a football goal post is referred to as a "goal post." It is intended that rotation about the X and Y axes is defined by the lens' position on the goal post. After the lens is mounted on the goal post, the goal post/lens assembly is then optically positioned along the Y and Z axes, and the lens affixed to the semiconductor laser diode. In order to perform these several alignments, a laser diode, usually the diode to which the lens will ultimately be assembled, is energized and the diode's laser beam directed through the lens to a screen. The operator manipulates the lens along and about the several axes until the projected beam meets the required specifications for the assembly. In this manner, movement along the several axes, as well as rotation about those axes is manipulated by an operator who assembles each lens and laser diode. The entire operation is very dependent on the skill of the operator, as the optical cement utilized first to affix the lens to the goal post and finally to the diode introduces a variable into the problem. This variable is engendered by the fact that the surface tension of the cement between the several elements on which it is used causes motion between those elements. This motion of course tends to misalign the optical elements. Active alignment methodologies are generally utilized to produce the devices taught in U.S. Pat. Nos. 5,081,639 and 5,181,224.
The term "passive alignment", as used herein, defines a process whereby the lens is aligned with respect to another device solely by mechanical means and thereafter secured in position with respect to the diode or other device. Examples of such mechanical means include mechanical jigs, fixtures, alignment blocks, and the like. Passive alignment does not require the projection of a beam of light through the lens, nor indeed manipulation of the lens with respect to beam alignment or performance. Passive alignment relies solely on the mechanical alignment of the lens with respect to the diode or other device to achieve the required optical alignment.
The term "semi-passive alignment", as used herein, defines an alignment methodology whereby the lens is aligned with respect to another device along at least one degree of freedom solely by mechanical means, i.e., passively. Examples of such mechanical means include mechanical jigs, fixtures, alignment blocks, and the like. Passive alignment does not require the projection of a beam of light through the lens, nor indeed manipulation of the lens with respect to beam alignment or performance. Passive alignment relies solely on the mechanical alignment of the lens with respect to the diode or other device to achieve the required optical alignment. Alignment with respect to one or more of the other degrees of freedom, where required, is effected by an active alignment scheme. The passive and active alignment steps in a semi-passive alignment methodology may be performed in any order.
Preferably, an ideal semi-passive alignment scheme performs the passive portion of the alignment along the most critical degree of freedom. This is often the alignment along the Z-axis. After all alignment is completed, the lens is secured in position with respect to the other device.
Significant effort has been expended to overcome the effects of pulling errors on the manufacturing efficiency of devices incorporating microlenses and other optical devices or elements. In general these methodologies, and the apparatus which perform them, can be divided into two classes: those which seek to increase the efficiency of the previously discussed active alignment process, and those which seek to achieve a passive alignment between the microlens and other optical elements.
While the method taught in U.S. patent application Ser. No. 08/725,151 spreads the active alignment "cost" over a number of devices, there still exists the need to actively align the diode row and the microlens taught therein. What is clearly needed is a methodology which will result in further substantial savings in skilled manpower currently required to accurately assemble a cylindrical microlens using current non-passive alignment methodologies. This advantage could be effected if some workable passive or semi-passive alignment methodology were made possible.
What is further needed is a methodology which enables the previously discussed passive or semi-passive alignment of a lens, particularly a cylindrical microlens, with respect to an electron device, particularly a laser diode, to less than 2 microns with respect to one or more degrees of freedom, most particularly along the Z axis of the microlens.
What is still further needed is a methodology which effects passive alignment while being relatively insensitive to changes in final microlens size resulting from pulling errors.
What is yet further needed is a methodology which scales its alignment with changes in size from one microlens to another.
The several references made herein to reference works and to issued and pending patents is to show the state of the art at the time the present invention was made. These references are herewith incorporated by reference.